The present invention relates to an electric system for an electric vehicle with a battery as a power supply. The battery may be a secondary battery, e.g., in a hybrid propulsion system.
A conventional system of this type is illustrated by FIG. 8 which shows an electric system with an AC motor for driving two wheels of a vehicle, and circuitry to power the motor and to provide for recharging of the battery during braking. The system includes a main battery 1, a fuse 2, a main circuit switch 3, an inverter 4, an AC motor 5, a differential gear 6, wheels 71 and 72, an initial charging circuit 8 for charging an input smoothing capacitor of the inverter, and a rheostatic braking circuit 9'. The charging circuit 8 includes an initial charging switch 81 and a charging resistor 82.
The inverter 4 typically consists of a three-phase transistor inverter as shown in FIG. 9, comprising a transistor 401 and a diode 402 connected in reverse-parallel to the transistor 401, together forming a switching arm. The three-phase inverter comprises six such switching arms.
An input smoothing capacitor 403 for inputting voltage to the inverter is connected to inverter 4 to smooth current from the main battery 1.
In operation of a conventional system, with reference to FIGS. 8 and 9, the main circuit switch 3 operates or stops the electric vehicle depending on whether it is switched on or off, and protects the main circuit. The fuse 2 further protects against damage that may not be prevented by the main circuit switch 3.
Since the input smoothing capacitor 403 is connected on the DC input side of the inverter 4 when the main circuit switch 3 is switched on to charge the capacitor 403 from the main battery 1 upon starting the inverter 4, the voltage of the capacitor 403 becomes twice that of the main battery 1 due to the inductance of the power line of the main circuit.
As a countermeasure, an initial charging circuit 8 is included. When the inverter 4 is started, the initial charging switch 81 is switched on while the main circuit switch 3 remains switched off. This results in charging of the input smoothing capacitor 403 for the inverter via the charging resistor 82. The resistance of the resistor 82 is selected so as to prevent resonance between the resistor 82 and the input smoothing capacitor 403 during initial charging. Thus, initial charging does not raise the capacitor voltage above the main battery voltage.
In the system of FIG. 8, the inverter 4 converts the DC power of the main battery 1 into AC power to control the torque and the speed of the AC motor 5. By the differential gear 6, torque is transmitted at reduced speed to the right and left wheels 71 and 72. In the motoring mode of the electric vehicle, the inverter 4 converts battery DC power to motor AC power, to drive the wheels 71 and 72, and thus to propel the vehicle.
Typically, in a braking mode of the electric vehicle, regenerative braking is used for more efficient use of the main battery 1. During braking as contrasted with motoring, the inverter 4 serves for AC to DC power conversion in regenerating kinetic energy of the vehicle via the wheels 71 and 72 and the motor 5 as DC power to the main battery. During such braking, the motor 5 functions as an electric generator.
An electric vehicle requires a braking performance comparable to a vehicle with an internal combustion engine. In particular, when going downhill, electric braking should be equivalent to engine braking.
This poses no difficulties so long as the main battery 1 can absorb the braking power. When the energy capacity of the battery is reached, however, regenerative braking fails.
For this eventuality, a rheostatic braking circuit 9' is included with the main circuit, as shown in FIG. 8. Since the rheostatic braking circuit 9' typically comprises a semiconductor power converter and a braking resistor so that it can alter the electric braking power according to the applied braking force, the braking system is complex and expensive.
During rheostatic braking in the electric system in FIG. 8, the power and current controlled by the power converter is applied to the braking resistor in the rheostatic braking circuit 9', and electromagnetic noise from the rheostatic braking circuit 9' may interfere with the vehicle's radio or telephone.
The following are design criteria for an improved electric system and equipment for an electric vehicle: (1) small size and light weight, (2) low manufacturing costs, (3) low maintenance requirements, (4) high system efficiency, (5) operability comparable to a vehicle with an internal combustion engine, and (6) low radio-frequency noise emission from the equipment and its power line.